As our understanding of the nervous system and its related disorders increases, a wider range of therapeutic and diagnostic agents will become available. Once these agents have been identified, it will be necessary to deliver them to sites of diseased tisssue in the central nervous system. Unfortunately, the existence of the blood-brain barrier limits the free passage of many types of molecules from the blood to cells of the central nervous system.
The physiological basis for the blood-brain barrier is the brain capillaries, which are made of endothelial cells (Goldstein, et al., Scientific American, 255: 74-83 (1986); Pardridge, W. M., Endocrin. Rev. 7:314-330 (1986)). These endothelial cells are different from those found in other tissues of the body. In particular, they form tight junctions between themselves. The actual blood-brain barrier is formed by these high-resistance tight intercellular junctions which form a continuous wall against the passive movement of molecules from the blood to the brain. These cells are also different in that they have few pinocytotic vesicles, which in other tissues allow somewhat unselective transport across the capillary wall. In addition, continuous gaps or channels running through the cells which would allow unrestrained passage are absent.
One function of the blood-brain barrier is to protect the brain from fluctuations in blood chemistry. However, this isolation of the brain from the bloodstream is not complete. There does exist an exchange of nutrients and waste products. The presence of specific transport systems within the capillary endothelial cells assures that the brain receives, in a controlled manner, all of the compounds required for normal growth and function. The obstacle presented by the blood-brain barrier is that, in the process of protecting the brain, it excludes many potentially useful therapeutic and diagnostic agents.
There are several techniques that either physically break through the blood-brain barrier or circumvent it to deliver therapeutic or diagnostic agents. Among these are intrathecal injections, surgical implants, and osmotic techniques.
Intrathecal injection administers agents directly into the brain ventricles and spinal fluid by puncturing the membranes surrounding the brain. Sustained dosages of agents directly into the spinal fluid can be attained by the use of infusion pumps that are implanted surgically. These spinal fluid delivery techniques are used to treat brain cancers, infections, inflammation and pain, but only penetrate into a minute fraction of the brain due to diffusion gradients and the density of neural tissues.
Clinicians prefer to avoid intrathecal injections because they frequently are ineffective and can be dangerous. Substances injected intrathecally are distributed unevenly, slowly and incompletely in the brain. Since the volume of the spinal fluid is small, increases in intracerebral pressure can occur with repeated injections. Furthermore, improper needle or catheter placement can result in seizure, bleeding, encephalitis and a variety of other severe side effects.
Clinicians also can administer agents that "crack open" the endothelial cells that line the brain capillaries. Dr. Edward Neuwelt at the University of Oregon uses such a system to deliver chemotherapeutics and imaging antibodies to tumors in the brain. (Neuwelt, E. A., Implication of the Blood-Brain Barrier and its Manipulation, Vols 1 & 2, Plenum Press, N.Y. (1989)) This technique involves an arterial injection of up to a 300 milliliter bolus of a 2 5% mannitol solution. The osmotic differential exerted by the mannitol causes the endothelial cells forming the barrier to shrink, opening gaps between them for a brief period. During this period, the drug is administered into the arterial system and is carried directly into the brain. The osmotic approach demonstrates that once past the barrier, therapeutics can be effectively distributed throughout the brain.
Because of the many risks involved, a 24- to 48-hour period in an intensive care unit is necessary following osmotic treatment. Mannitol can cause permanent damage (including blindness) to the eye. If the barrier is permeable for too long, edema results. Cells of the brain also can be damaged when neurotoxic substances in the blood, not generally accessible to the brain, are able to cross the barrier. Finally, there is a serious incidence of seizures in patients during and after the procedure.